1. The Field of the Invention
The present invention relates to the field of high-speed data communication. In particular, the present invention relates to a circuit and method for reducing electromagnetic interference in a data communication system.
2. Description of the Related Art
In high speed optical communication systems, laser signals can be used to transmit information. For example, a laser driver circuit is used to drive and modulate the amount of electric current to be received by a laser diode. The laser diode emits a laser signal in response to the magnitude of the electric current received.
A conventional laser driver circuit may be implemented with a plurality of differential amplifiers consisting of pairs of transistors for driving a pair of differential current output signals. A problem with the conventional laser driver circuit is that it generates undesirable common mode glitches as the transistors of differential amplifiers switch states from on to off or vice versa. The common mode glitches may reverberate within the laser driver circuit, causing the circuit to radiate electromagnetic noise. These common mode glitches are difficult to terminate within an integrated circuit. One potential solution is to use inductors external to the laser driver integrated circuit to terminate the output signals. However, external inductors increase the cost of the system and occupy valuable circuit board space. Hence, there is a need for a laser driver circuit that can minimize common mode glitches when the differential amplifiers of the laser driver circuit transition between states.
A conventional laser driver circuit is slow in draining the electric charges accumulated in the differential amplifier circuits and therefore results in a slow laser turn off performance. One of the design goals of the laser driver circuit is to ensure a fast signal transition for the pair of differential current output signals, especially for the high to low transition for turning off the laser diode. This design goal may be accomplished using differential amplifiers having high amplification gains in order to ensure a fast signal transition. However, having high amplification gains creates signal overshoots when the output signals transition from low to high, and the overshoots cause undesirable electromagnetic noise.
Another design goal of the amplification circuit is to minimize the electromagnetic interference created by the laser driver circuit. This design goal may be accomplished by using differential amplifiers having low amplification gains to drive the output laser signal. As a result, a design tradeoff has to be made to choose between having high or low amplification gains in the differential amplifiers, and the design is compromised because it has to choose between the adverse effect of electromagnetic interference or a slower signal transition. The adverse effect of electromagnetic interference may lead to additional system costs employed to reduce the interference problem. The adverse effect of slower signal transition may lead to lower system performance. Therefore, there is a need for a laser driver circuit having a high amplification gain for signal transition performance and at the same time reduces the adverse side effect of electromagnetic interference.